Over the past ten years digital radiation imaging has gradually been replacing conventional radiation imaging where the recording means is film or an analog device such as an Image Intensifier. Currently, several such devices are available that can perform digital radiation imaging. In some cases, incident radiation is detected and converted locally into an electronic signal which is then collected at collection/pixel contacts and then further transmitted to readout circuits which perform various functions including digitization. In other cases, the radiation is detected and converted into light which is then converted to an electronic signal and subsequently is readout and digitized. The first cases we refer to as “direct radiation detection,” and the second cases we refer to as “indirect radiation detection.”
Direct radiation detection devices typically comprise a semiconductor detector substrate conductively bonded to a semiconductor readout substrate. The detector substrate is made of a photo-conductor material which converts incoming radiation into electronic signals. The readout substrate accumulates such electronic signals, processes them and reads them out. There are different kind of photo-conductor substrate technologies and different readout substrate technologies. Table I broadly summarizes various types of direct radiation digital imaging technologies, and lists typical cases in each technology group.
The following terms as used herein have their standard meaning in the electronics literature: CCD stands for Charge Coupled Device, ASIC stands for Application Specific Integrated Circuit, TFT stands for Thin Film Transistor array. Detectors are materials or devices whose response to X-ray energy is used to indicate the presence or amount of radiation incident on the detector. X-rays are electromagnetic radiation lying in a range between “cosmic rays” and “ultraviolet rays.” This range is defined as lying between 0.001 and 100 angstrom units or 10−11 and 10−6 centimeters in wavelength. As used herein, the term “gamma ray” is considered to be synonymous with the term “X-ray.” Gamma rays are usually considered to be produced by some natural phenomenon such as the decay of an atomic nucleus whereas X-rays are usually considered to be produced by an electronic tube or other manufactured device.
TABLE IRadiant Energy, Direct Digital Imaging TechnologiesDetectorReadoutSubstrateTechnologySubstrateSubstrateInterfaceSBBASICCdTe; CdZnTe;CMOS; BiCMOS;Bump-bondsSi; Ge; GaAs;HBIMOS; SiGe;TlBr; PbI; MgI;Mixed Signal/RF;etc.Logic; etc.a-SGTFTa-Se; a-CdZnTe;a-Si:H TFTEpitaxial growth;a-CdTe; etc.Evaporation; etc.a-SGASICa-Se; a-CdZnTe;CMOS; BiCMOS;Epitaxial growth;a-CdTe; a-PbI;HBIMOS; SiGe;Evaporation; etc.etc.Mixed Signal/RF;Logic; etc.;Abbreviations:SBBASIC = Semiconductor Bump Bonded on ASIC;a-SGTFT = amorphous Semiconductor Grown on TFT;a-SGASIC = amorphous Semiconductor Grown on ASIC.
Digital radiation imaging devices utilizing SBBASIC technologies are known in the art, and typically comprise a crystalline detector semiconductor substrate (photo-conductor) and a semiconductor readout substrate incorporating integrally processed ASICs. The detector and readout substrates are joined together and electrically communicate by means of bump-bonds or other conductive means. The detector substrate has a continuous electrode on a first major face (where incident radiation impinges) and a two dimensional array of charge collecting/pixel contacts or electrodes on a second major face, opposite the first major face. Incident radiation is absorbed in the material of the detector substrate and electrical charge is generated in response to such absorption. Under the bias of an electric field between the first and second faces, the generated charge drifts toward and is collected at the charge collection/pixel contacts or electrodes. Each charge collection contact defines a separate “pixel” on the detector substrate and is conductively connected to a corresponding “pixel circuit” on the readout substrate by a bump-bond. Each pixel in combination with its corresponding pixel circuit comprises a “pixel cell.” Each pixel circuit on the readout substrate may include various circuit features for amplifying, storing, digitizing, etc. the incoming charges. The bump-bonds may be accomplished using a variety of metals or compounds including various solder alloys and other conductive compositions.
Typically, at a perimeter edge of each readout substrate there is at least one region for routing input and output (I/O) signals to and from the readout substrate. These can be wire bonding pads or similar features for providing electrical connections to the ASICs of the readout substrate.
Kramer el al., U.S. Pat. No. 5,379,336, disclose a typical SBBASIC device, see FIGS. 1A and 1B. As shown in the figures, a semiconductor detector substrate 10 is bump bonded with an array of conductive bumps 13 to a readout/processing substrate 12. Both the semiconductor detector substrate 10 and the semiconductor readout/processing substrate 12 are each integral and monolithic. Examples of detector and readout/processing substrate technologies is given in Table I. Radiation hv is incident on the top (first major face) of the detector semiconductor substrate 10. A pixel array is formed by means of metal charge collection/pixel contacts on the exit face (second major face) of the detector semiconductor substrate 12. Electrical charge created in the semiconductor detector substrate 10 in response to absorption of incident radiation hv is collected by the detector pixel contacts 14. The collected charge is communicated through the conductive bumps 13 to corresponding pixel circuit contacts 15 on the readout/processing semiconductor substrate 12. The pixel circuits are used to perform a variety of possible functions including accumulating the incoming charge and/or amplifying it, discriminating, digitizing, counting incomings radiation hits, etc.
Orava et al., U.S. Pat. No. 5,812,191 and Spartiotis et al., U.S. Pat. No. 5,952,646, both disclose alternative embodiments of an SBBASIC-type digital radiation imaging devices. In these imaging devices as generally exemplified in FIG. 2, the detector semiconductor substrate 30 is electrically connected to the readout semiconductor substrate 32 with bump-bonds 35. The photo-detector material 34 of the semiconductor substrate pixels 36 absorbs incoming radiation, and in response to the absorption generates electrical charges. The electrical charges are collected at collection/pixel contacts 38, and electrically communicated through the bump-bonds 35 to the pixel circuit contacts 33 on the pixel circuit 31 of the readout semiconductor substrate 32.
However, the above noted SBBASIC imaging devices are unitary devices with an imaging area that is limited by current semiconductor manufacturing and bump-bonding technologies. At present, some of the most sensitive photo-conductor materials, such as CdTe, CdZnTe, TlBr, PbI, and GaAs, can be used to manufacture single crystal semiconductor substrates without defects having dimensions of only about 3″ or 4″. Imaging area is even more limited with the CMOS technology typically used to create the semiconductor readout substrates. These technologies typically can produce radiation imaging devices having active imaging areas of at most a few square centimeters. Even if semiconductor substrate dimensions are increased, current bump-bonding technology would still limit the planar area of the detector and readout substrates that can be bonded together (e.g., a 10 cm×10 cm monolithic detector substrate to its readout substrate). An additional concern for the bonding of the detector and readout substrates together is the flatness of the substrates and the uniformity of the conductive bump needed to accomplish the process.
In view of these limitations, the field has been motivated to develop technologies that make it possible to industrially perform high density bump-bonding operations between single semiconductor substrate pairs. For example, “tiling” techniques have been developed in which a plurality of digital radiation imaging device units are “tiled” together in a one or two dimensional array to form a larger imaging device mosaic. Tiling of individual digital imaging devices allows production of digital radiation imaging devices having much larger imaging areas. However, tiling techniques have also introduced an amount of imaging dead area into the imaging area of the mosaic imaging device, which can adversely affect device's image quality. This imaging dead area is primarily resultant from the planar area of an individual digital imaging device tile that is required to provide the I/O connections to the individual device, e.g., the wire bonding area. Even though the depth of the wire bonding area is typically a few mm, it can create an imaging dead area that is unacceptably large for a particular radiation imaging application.
Therefore, the field has been further motivated to develop tiling techniques that reduce the amount of dead area in a mosaic or arrayed digital imaging device. FIGS. 3A and 3B illustrate an early attempt from Lemercier et al., EP 0421869, to reduce the wire bonding area 61 and other possible edge-most inactive or imaging dead areas 61a on one or two sides of an SBBASIC by overlapping some of the imaging dead areas 61 with active detector area 62 in a “stair case” arrangement of individual SBBASIC tiles. The whole “stair case” is mounted on a support 60. Although this technique does reduce the total amount of imaging dead area of an imaging device array, some perimeter dead areas 61 still remain. Additionally, in order to maximize image quality, the surfaces of all the individual SBBASIC tiles must be parallel to each other. This is mechanically difficult in the production of imaging devices like the Lemercier device. Further, the “stair case” approach required that to achieve larger the imaging areas, the support substrate 60 must be made relatively thicker in two directions.
In order to overcome the limitation of needing a double-ramped support substrate as in the Lemercier device, the field has developed alternative tiling techniques. One example shown in FIG. 4 is that of Schulman, EP 1162833, whereby imaging device tiles 56 & 58 are removably mounted on a support board. On one edge of the readout substrate 52 there is an imaging dead area 50 which extends beyond detector substrate 51 of the device tile 56 or 58 and is reserved for wire bonding. The wire bonding area is not sensitive to radiation and does not perform imaging. Each SBBASIC is mounted on a combination wedge 44 and platform 53, which in turn is mounted on a PC board 54. The wedge-platform combination allows the inactive area 50 of one imaging device tile 58 to at least partially go under the active imaging area another imaging device tile 56. This technique is complicated in its execution because the wedge-platform requires careful control of the tilt angle and precise alignment of the tile devices relative to each other. Further more, the inactive area is never completely covered and the tile angle can introduce a parallax error depending on the angle of incidence of the incoming radiation.
While the SBBASIC technology is relatively the newest approach to direct radiation digital imaging and has advantages over the other prior radiation digital imaging technologies, it also currently has certain limitations:    a. In current devices, the detector and readout substrates are manufactured with a limited field of view. Field of views of single devices of only up to 2.5 cm2 have been reported. This is insufficient for most commercial applications.    b. Due to this limitation, tiling techniques that combine a plurality of SBBASIC devices have been suggested to provide a larger field of view. However, such tiling techniques can be cumbersome and difficult to implement on an industrial production scale. This can adversely impact the quality of imaging and the cost of the complete camera head comprising a plurality of such SBBASICs.    c. In addition to the limitation of (a) and (b) above, in current SBBASIC imaging devices, the interconnection of the ASIC with wire bonding pads introduces an “inactive” area for each SBBASIC device. This is an area that is not useable to image incoming radiation. Such “dead” areas adversely impact image quality, especially when they are too large to cancel them out by software.
Although each of the above radiation imaging devices may be useful for their intended purposes, it would be beneficial in the field to have an alternative radiation imaging device that eliminates or further minimizes imaging dead area due to wire bonding requirements of the ASICs involved, without requiring a support ramp. Additionally, it would be beneficial to have the semiconductor tiles mounted in the same plane. It would be further beneficial if the device can be produced using current bump-bonding techniques in combination with the new high sensitivity semiconductor materials that can be mechanically brittle and susceptible to relatively high bumping temperatures.